Aviation has become the major form of transportation in today's world. Airplanes and helicopters are now the predominant means of travel for a substantial sector of our society, and the use of aircraft by commercial carriers to transport mail, business documents, parcels and critical goods has proliferated.
As reliance upon air transportation increases, pilots are called upon to fly their aircraft under a wide variety of weather conditions. Because aircraft performance is highly dependent upon the atmospheric conditions at the time of the flight, unfavorable or even dangerous conditions may be experienced.
One of the most dangerous conditions to which pilots are exposed is the accumulation of ice on the various surfaces of the aircraft. Carburetor and induction icing can suffocate a reciprocating engine, and icing at the intake of a jet engine can markedly reduce the power available. The build-up of ice on the airfoils which sustain flight have an adverse affect that compounds itself. That is, as ice accumulates on the airfoil it changes the aerodynamic configuration of the airfoil which can reduce the lift and increase the drag. At the same time the accumulated ice increases the weight which the airfoil must support. To sustain flight under these conditions the engine(s) must produce more and more thrust, an interim solution which reduces engine life and consumes considerable fuel. Eventually, severe ice accretion will prevent the aircraft from maintaining altitude, and it can only be hoped that the airfoil will continue to produce enough lift to permit a controlled descent until a forced landing can be safely completed. The chances for safely concluding such a flight without adequate means to prevent, or remove, ice accretion are recognized as being minimal.
An accretion of ice on the propeller of an airplane or the rotors of a helicopter similarly reduces the aerodynamic lift--i.e., thrust--provided by those components, again seriously degrading the expected flight characteristics. In addition, it should be noted that if the ice forms unevenly, or should, for some reason, the accumulated ice happen to be thrown, or be removed in whole or in part, from one blade only, the imbalance so created can result in such severe vibrations that: the propellor, or rotor, can itself be destroyed; the engine can be torn from its mounts; or, any component in the drive train from the engine to the propellor, or rotor, can fail.
For these reasons federal regulations require that an aircraft be specially certified to fly into known icing conditions. That is, the aircraft must be equipped with satisfactory means to obviate disastrous ice accretion while the aircraft is in flight.
To combat the effects of ice, aircraft manufacturers have developed various methods either to prevent the formation of ice or to limit and control the ice accumulated. These methods include pneumatic de-icers, which dislodge accumulated ice, alcohol sprays, which prevent formation of ice (an anti-icer), and electrical heating elements which serve as both anti-icers and de-icers.
The problem which faces aircraft manufacturers is where, and how much, de-icing equipment should be installed. If too little equipment is installed, or even if the proper amount of equipment is installed but not in the proper areas, the equipment will be ineffective in preventing or removing ice accumulation. On the other hand, if more de-icing equipment is installed than is necessary, the aircraft is penalized with excessive weight and power consumption as well as additional cost.
To date the amount of de-icing equipment to be used, and its location along the airfoil, has been selected on the basis of years of experience and limited empirical data. Typically, a prototype installation is subjected to test flights conducted under artificial and natural icing conditions to determine if adequate protection had been achieved. Such trial and error approaches are generally expensive. If the prototype operates satisfactorily, overdesign is seldom appreciated, and the system will continually penalize the particular aircraft model with unnecessary weight and cost. If the prototype operates unsatisfactorily, one or more modifications must be incorporated in the prototype before satisfactory performance is achieved. Moreover, test flights with unsatisfactory de-icing equipment can easily become a dangerous experience.
It has heretofore been demonstrated that the dimensions of the crater formed by the impingement of a water vapor droplet on a gelatinous surface is, at least within a given range of airspeeds and impact angles, directly related to the size of the water vapor droplet that produced the crater. It has not, however, been heretofore recognized that such knowledge could be applied empirically to determine the icing field limits on complex shapes subjected to laminar and/or turbulent airflow.